A number of low cement castable refractories have been developed for use within the aluminum industry. These castable refractories consist of a blend of calcined aggregate, cements and fillers which are mixed in various proportions to suit the specific end applications. The calcined aggregate is selected from a range of materials, including tabular alumina, bauxites, kaolins and other clays, and normally contributes up to 95 wt % of the final refractory mixture.
Characteristics and properties of low cement castables compared with conventional castable refractory materials include:
(i) A low moisture requirement during mixing. This results in a fired refractory of higher density and lower porosity.
(ii) Higher cured and fired strengths, particularly in the temperature range of 600.degree. to 1100.degree. C.
(iii) Greater chemical and erosion resistance against molten metals and slags.
(iv) Higher refractories and greater thermal shock resistance.
Even though superior to conventional castables, a major disadvantage of the low cement castables currently available is their susceptibility to chemical attack and erosion. This necessitates a relatively frequent furnace or crucible shutdown/replacement program and hence reduced productivity. Where used as a barrier layer in reduction cells, a gradual breakdown of the refractory over time leads to exposure of the underlying insulation layers to the aggressive cell environment. Ultimately this will result in cell failure and shutdown, but prior to this a gradual increase in cell voltage and hence a resulting loss in energy efficiency.
A further disadvantage of low cement castables presently available is their high cost compared with conventional castable refractories. This is largely an artifact of the aggregate material; premium grade calcined minerals have been used exclusively for these applications.